The equation expressing the dissolved oxygen balance in a flowing stream is formulated to include a time-varying photosynthetic oxygen source. The solution to this equation is compared to diurnal dissolved oxygen data obtained from five streams for which the algal oxygen source is the primary cause of the diurnal dissolved oxygen fluctuation. The importance of the nitrogenous component of BOD is also evident. The resulting agreement between the theoretical solution and the data indicates that the equation presented is an adequate representation of the dissolved oxygen balance in a time-varying as well as spatially varying situation.